During the summer months, the temperature inside an automotive passenger compartment can rise substantially; especially, when the car is parked outside and the sun's heat radiation penetrates steadily through the windows. In order to stabilize the interior temperature while driving the car, many car models are equipped with air-conditioning systems. But providing a sufficient cooling capacity requires a high energy expenditure.
On the other side, during the winter months, the driver and the passengers are often confronted with very low temperatures in the passenger compartment at departure. After turning on the heating system, it usually takes a few minutes before a significant temperature increase occurs. In order to solve the problem, some car models have extra heating systems installed in their seats. The energy necessary for running the seat heating system is provided by the car's battery. Due to addition of new auxiliary systems for monitoring and controlling functions in the cars over the past years, the power supply needed for their operation has steadily increased. In order to prevent demands for further increases in the battery's capacity, energy savings are necessary.
Phase change material possesses the ability to change its physical state within a certain temperature range. When the melting temperature of a phase change material in a heating process is obtained, the phase change from the solid state to the liquid state occurs. During this melting process, the phase change material absorbs and stores a large amount of latent heat. The temperature of the phase change material remains nearly constant during the entire process. In a cooling process, the heat stored by the phase change material is released into the environment in a certain temperature range and a reverse phase change from the liquid state to the solid state takes place. During this crystallization process, the temperature of the phase change material also remains constant. The high heat transfer during the melting process and the crystallization process, both without any temperature change, is responsible for the phase change material's appeal as a source of heat storage.
In order to contrast the amount of latent heat absorbed by a phase change material during the actual phase change with the amount of sensible heat absorbed in an ordinary heating process, the ice-water phase change process will be used. When ice melts, it absorbs an amount of latent heat of about 335 J/g. When the water is further heated, it absorbs a sensible heat of only 4 J/g while its temperature rises by one degree C. Therefore, the latent heat absorption during the phase change from ice into water is nearly 100 times higher than the sensible heat absorption during the heating process of water outside the phase change temperature range.
In addition to ice (water), more than 500 natural and synthetic phase change materials are known, including salt hydrates, metals, alloys, poly-alcohols, eutectics and paraffine. These materials differ from one another in their phase change temperature ranges and their heat storage capacities.
In the present applications of the phase change material technology in garments and home furnishing products, paraffine are used exclusively. These are crystalline alkyl hydrocarbons with different chain lengths. Characteristics of suitable paraffine are summarized in Table 1.
TABLE 1Thermal characteristics of selected paraffineLatentMeltingCrystallizationheat storageParaffintemperature, ° C.temperature, ° C.capacity, J/gHeneicosane40.535.9213Eicosane36.130.6247Nonadecane32.126.4222Octadecane28.225.4244
Compared to other phase change materials, the paraffine possess very high heat storage capacities. Furthermore, paraffine can be mixed in order to realize desired temperature ranges in which the phase change will take place. Paraffine are nontoxic, noncorrosive and nonhygroscopic. The thermal behavior of the paraffine remains stable also under permanent use. Paraffine are byproducts of petroleum refining and therefore inexpensive. A disadvantage of the paraffine is their low resistance to ignition. But this problem can be solved by adding fire retardants.
In the present applications, the paraffine are microencapsulated. They are applied to a textile matrix by incorporating them into fibers and foams or by coating them onto a textile surface. For example, microencapsulated paraffinic phase change materials have been described as a suitable component for substrate coating when exceptional heat storage capabilities are desired. The U.S. Pat. No. 5,366,801 for “Fabrics with reversible enhanced thermal properties” to Bryant, et al., incorporated herein by reference, teaches that substrates coated with a binder containing microencapsulated phase change material enables a substrate to exhibit extended heat storage properties. Furthermore, microencapsulated phase change materials have been described as a suitable component for inclusion in fibers or foams. U.S. Pat. No. 4,756,958 for “Fiber with reversible enhanced thermal storage properties and fabrics made therefrom” to Bryant et al., incorporated herein by reference, teaches that a fiber with microencapsulated phase change material possesses enhanced thermal storage properties. U.S. Pat. No. 5,637,389 for “Thermally enhanced foam insulation” to Colvin et al., also incorporated herein by reference, reports a thermally enhanced foam insulation where the microcapsules are embedded within the foam.
There are also compound structures with incorporated phase change material available now which might be more appropriate for some automotive interior applications. U.S. Pat. No. 4,908,166 for a “Method for preparing polyolefine containing a phase change material” to Salyer, incorporated herein by reference, teaches that a polyolefine material containing phase change material possesses enhanced thermal storage properties.
There are several thermal effect which can be obtained by a phase change material application in a certain product, such as:                a cooling effect, caused by heat absorption of the phase change material;        a heating effect, caused by heat emission of the phase change material;        a thermo-regulating effect, resulting from either heat absorption or heat emission of the phase change material which keeps the temperature of a surrounding substrate nearly constant.        
The efficiency and duration of each of these effects is determined by the heat storage capacity of the phase change material, the phase change temperature range, the structure of the matrix material carrying the phase change material, and the structure of the end-use product.